1. Field of the Invention
The present invention relates generally to servo motor control devices for electric injection molding machine, and more particularly, to a servo motor control apparatus for an electric injection molding machine having synchronous function.
2. Description of the Related Art
In general, electric injection molding machine employs a single servo motor as drive source for injection molding. For a large scale electric injection molding machine which requires a large output power, the method of upgrading the output power of the single servo motor usually gives rise to the problems of increasing the costs and complicating the manufacture process. Accordingly, in practice, a plurality of small output power servo motors are employed in parallel to cooperatively drive the injection screw of the electric injection molding machine, which achieves an identical effect with the employing of a large output power servo motor. Referring to FIG. 3, which illustrated a schematic diagram showing an outline construction of an electric injection molding machine, wherein a servo motor control apparatus 61 controls and drives a master servo motor 62 and at least one slave servo motor 63. Mechanical connecting means, for example a belt 64, is employed to interconnect the master servo motor 62 and the slave servo motor 63 for synchronously driving an injection screw 65, further injecting melted resin in a barrel 66 to a cavity 67 for molding.
Consequently, how to well design the control apparatus 61 to thereby control the synchronous operation of the plurality of servo motor 62, 63 and reduce the problems of interference and vibration caused by non synchronous operation, has become a key step for improving the function of the large scale electric injection molding machine 60. Many conventional technologies has brought forward different control apparatus for improving the injection effect. Referring to FIG. 4, which illustrates an inner function block diagram of a conventional servo motor control apparatus, wherein the conventional servo motor control apparatus employs a most basic control method that employs a position command generator 77 in a motion controller 76 to generate a position command, wherein the same command synchronously drives two drive amplifiers 78 and their corresponding servo motors 79, in which, the servo motors 79 are parallel to one another, and no master or slave servo motor are defined. However, using the same position command for synchronously controlling fails to achieve an accurate synchronous effect due to many variable factors that may affect the rotation of the servo motors, such as the electrical parameters difference of different servo motors, the friction between different mechanisms, the frictional resistance in different positions or the different characters of the new components and the old ones. Hence, even if different servo motors are being actuated with the same position command, it may still result in a non synchronous outcome due to the aforementioned factors. Further, when said conventional servo motor control apparatus is used, a mechanical connecting means, for example a belt, must be employed to the output shaft of the two servo motors for diminishing the difference of the rotation angular positions between the two servo motors 79. Thus the unreliable synchronous effect may chance to induce interference and damage to the mechanical connecting means.
Referring to FIG. 5, an inner function block diagram of another conventional servo motor control apparatus is shown. Said conventional servo motor control apparatus controls a master servo motor 71 via a position command, while the master servo motor 71 controls a slave servo motor 72 via a speed command. Said conventional servo motor control apparatus is configured such that the slave servo motor 72 is synchronously controlled in accordance with the rotation speed of the master servo motor 71. However, the problem of mechanical connecting means, for instance, a belt employed to connect the master drive mechanism and the slave drive mechanism, which may induce system hunting because of external disturbance, still exists in said conventional servo motor control apparatus.
Referring to FIG. 6, U.S. Pat. No. 6,142,760 which discloses a control apparatus. The control apparatus controls a master servo motor 81 via a position control mode, while the master servo motor 81 controls a slave servo motor 82 via a torque control mode, thereby driving the slave servo motor 82 synchronously with the master servo motor 81 to improve the problem of system hunting. However, non synchronous effect caused by different frictional resistance may occur in said control apparatus, for example, in the case of the master servo motor 81 increases the torque output thereof in response to a large frictional resistance, while the slave servo motor 82 which does not suffer frictional resistance also increase its torque output in response to the action of the master servo motor 81, thus the rotation speed of the slave servo motor 82 is greater than that of the master servo motor 81. As a result, the mater servo motor 81 can not synchronously cooperate with the slave servo motor 82, which likely induces interference and damage to the mechanical connecting means of the two servo motors.
Additionally, FIG. 7 illustrates an inner function block diagram of another servo motor control apparatus disclosed in U.S. Pat. No. 6,046,566. The servo motor control apparatus controls a master servo motor 91 via position control mode, and controls a slave servo motor 92 via speed control mode. The speed command of the slave servo motor comes from the speed of the master servo motor 91. A feedback of synchronous error adjusting is incorporated into the servo motor control apparatus for achieving a better synchronous adjusting effect. However, the problem of mechanical connecting means mentioned above still exists in this servo motor control apparatus, and what's more, the response speed of the synchronous adjusting is somewhat slow, which delays the synchronous adjusting of the slave servo motor 92, and can not satisfy the requirement of the synchronous drive efficiency.
Thus, how to develop a servo motor control apparatus for electric injection molding machine to synchronously control the rotation speed of multiple servo motors, thereby achieving an improved accumulative effect, simultaneously achieving a fast synchronous response and reducing vibration friction, has become an urgent task in this field.